Thermal Expansion
by Turbotail88
Summary: They can't control their desires, so their only option is to try in a... locker? As their passion elevates to new heights, so does the temperature. Futa, sex, AU, Ymir/Krista


It was a hot, summer afternoon when Krista glanced behind her and shivered at the look her girlfriend gave her. Ymir smirked and leaned forward, winking and placing her chin in her palm. The blonde blushed and turned away, struggling to write what her professor was saying and _not _what she was feeling. It was proving hard to concentrate on the topic of her history class however, what with the way Ymir was licking her lips and grinning.

Ever since they'd confessed to each other one cold autumn morning, under the swirling leaves of yellow and red, they rarely spent more than a day without talking or seeing one another. However, now that they were sexually active, Ymir became more possessive and affectionate while Krista struggled to keep up and sate her lover. It was endearing, yet often led to random places where they would end up making love in a fit of passion. This stayed on the forefront of Krista's mind, clouding her thoughts and making her squirm in her seat. She was anxious for school to be over so she could spend time with her lover.

"...and that is how the Nazis were pushed back by the Soviets during the invasion of the Soviet Union," Levi drawled, putting down his chalk. "Class, please write a summary on pages fifty to seventy-two for Monday. And do the questions, while you're at it. Eren, one more word and I'm going to give you all a test next week!"

"Yes, Mister Levi," the class chorused. Eren grumbled angrily in the background and crossed his arms when Mikasa patted his head.

"Class dismissed."

Ymir quickly stood and leaned against Krista's desk the second the blonde got up. The brunette watched as she gathered her books and smiled up at her. Ymir's heart swelled with emotion and she hugged her girlfriend tightly. Krista gasped but smiled warmly, closing her eyes and snuggling in her embrace.

"Finally," Ymir breathed, "I've been wanting to do that all class."

"Seriously?" Krista giggled, "I thought you wanted to do something else with the way you were looking at me.."

"Oh yeah, that too."

"Ymir.."

The tall girl barked out a laugh and stepped back. She snickered at the blush on the shorter girl's cheeks and tugged on her hand. Krista allowed herself to be dragged away by her lover and grinned at her enthusiasm. It wasn't often that Ymir was excited for something, and she was happy to see a grin on her face rather than a bored expression.

Ymir pulled her all the way to their lockers, which were suspiciously next to each other. They were tall and wide enough to fit one person, though two could squeeze in if they were positioned properly. They both placed their books in as they chatted lightly, sharing the happenings of the day and their plans for the weekend. Krista giggled at a joke Ymir recounted, and the brunette ruffled up her hair when she whined about the homework they had for their classes.

Their actions were deliberately slow and their secret glances obvious to only themselves. Krista packed her book bag with whatever she needed from school and Ymir fumbled with her school lock deliberately. Soon, the hallway emptied and only the sound of Krista shutting her locker remained. Both teens walked to school from their homes, so transportation was not an issue for either of them.

Ymir smirked and gently pushed Krista against her locker, trailing a hand down her side. Krista smiled and put her hands on her shoulders, looking up at her with happy blue eyes. The brunette sighed and lowered her head to initiate a long, passionate kiss that left them both breathless. Ymir pulled back and hooded her eyes, putting a hand to the blonde's cheek.

"I love you," she whispered, overcome by emotion.

"I love you, too," Krista murmured in response, taking the clip from Ymir's hair and placing it in her pocket. Suddenly, she grinned and touched the front of Ymir's uniform pants, making the brunette groan.

"Up already?" Krista giggled.

Ymir grumbled and her hips twitched forward when the blonde pressed her hand a bit harder, squeezing the member in her palm lightly. She took her hands and held them above the petite student's head with one of her own. Krista pouted, but closed her eyes when the freckled girl covered her pouting lips with a searing kiss that made her squirm. Their bodies pressed against one another and Ymir thrust her hips forward when the blonde moaned. She trailed her lips down her neck, biting and sucking along the way, however a sound in the distance froze her in her tracks.

_"Hey! Slow down!"_

"Shit.." Ymir muttered. Krista panted heavily and shut her mouth when Ymir put her finger to her lips. Krista's chest heaved and she assumed a pleading expression, wriggling in Ymir's hold. The brunette grit her teeth and looked around, cursing their luck. There wasn't a single classroom or supply room within a reasonable distance, and she could already hear the rapid footsteps approaching. She glanced at the lock digging into Krista's back and smirked. Quickly, she skillfully undid the lock and shoved Krista inside the locker. The blonde squeaked in protest but made no other comment when Ymir joined her and shut the locker behind her. She held it by the edge in order to prevent it from flying open and peeked from the tiny crack.

Sasha sighed and picked up the can of food that had somehow rolled away from her all the way from the kitchen. She stood for a moment, looking around, because she could've sworn she heard something. She shrugged, and with a smile, skipped back the way she came. She waved to Mikasa, who was pulling Eren by his ear across the hallway, and giggled to herself.

Ymir scowled when she saw the small stream of students walking by. It was Friday, meaning there were obviously going to be student council meetings, after school activities and other such disturbances. She gasped when a tiny hand shoved its way into her pants and gripped her cock tightly. After making sure the locker wouldn't move, she looked down to see the shred of light from the hallway being reflected in Krista's desperate eyes. She swallowed harshly at the look.

"Ymir, please, let's continue," Krista pleaded, pressing closer to her, "I'm.. I can't take it.. it's been so long.."

The brunette exhaled quietly through her nose and buried her face in her lover's neck. "Yeah, I know.. damn it's hot in here."

"We're in a locker."

"I know... and it's pretty cramped in here too - I just can't control where my hands wander.."

"What's that supposed to- oh."

Ymir snickered and squeezed her girlfriend's rear, earning a gasp and low moan. She trailed her hands up and under her shirt, caressing her back, before hooking her thumbs on her skirt and sliding it down. Krista tightened her hand on her cock in reproach, hiding a smile in the collar of Ymir's shirt. With a suppressed groan, the brunette captured her lover's lips in a kiss and pressed her harder against the metal wall behind her. Her neck craned under the shelf above her and she decided to make things quick before it snapped.

Lifting Krista up slightly in her arms, Ymir shoved her tongue in her mouth and her fingers tapped against the fabric of her panties. Krista whined slightly when her hands lost contact with the hard shaft but instead held Ymir's face in her palms. She kissed with as much force as she could, sighing into her lover's mouth and trying to meet her eyes through the low visibility they had. When she did finally gaze into those brown orbs, she shivered at the intense look, at the absolute love and lust. Krista pulled back and tried to keep a quiet breathing pattern. She ran her fingers through Ymir's damp hair and tugged slightly on her short locks.

"Ymir.." she gasped, "Ymir, I love you."

The brunette kissed her forehead and smiled genuinely in reply. Just as she opened her mouth, another sound cut through the haze of pleasure they'd been losing themselves in and made them both freeze in fear.

"What the hell is that noise!" Keith Shadis stomped down the hall in a fit of rage after being humiliated in the teacher's lounge by Levi (again). He whirled around, determined to find the source of the noise - and thus his venting tool - but for some reason stopped short. Ymir peeked through the crack, desperate to find out what was going on, but bit her lip. Krista had found a way to shove her hand down the front of her pants _again_ and had started to stroke her cock _just right_.

_"Krista!" _Ymir hissed.

"I need you, Ymir," Krista whimpered, biting her lip in such an adorable manner that it made Ymir's glare falter. "Only you.. only you can make me feel this way," Krista continued softly, "and I need you now." To emphasize her point, she moved her hand out of Ymir's pants and undid the button. Ymir trembled under her touch and tore her eyes away from the crack to watch Krista push her trousers and boxers to her ankles with both her hands. The brunette let out a hot breath when the blonde guided her cock to her entrance.

They forgot about Keith, about school and anything around them. They forgot that they were making love in a cramped locker, and the heat of their skin, the kink in Ymir's neck. The second Ymir entered Krista, her blood froze and she halted.

"Wait!" Ymir choked out quietly through the pleasure, "con-condom! We forgot.."

"It's fine, sweetheart," Krista murmured, "I'm on the pill."

Ymir leaned forward and pressed her forehead to the warm surface behind Krista, slowly sinking into her lover. They glued their lips together to silence their noises of pleasure. Ymir briefly heard the sound of footsteps distancing themselves as she pulled back and thrust in harshly. Krista cried out into her lover's mouth, but it was swallowed up by the skillful movement of her tongue. Ymir quickened her pace and their bodies started to bang against the interior of the locker. The heat was building up, the sweat pouring from their scalp and down their faces. Krista opened her eyes and lovingly traced the line of freckles as she pulled back to smile at her lover. Ymir grinned when Krista's smile faltered and she let out a loud moan.

There wasn't much room to allow deep thrusts, which gave Ymir the option of rolling her hips against Krista's and rocking against her body, something they both thoroughly enjoyed. Their shirts dampened with sweat and clung to their skin, but neither seemed to mind as Ymir nipped at Krista's neck and rocked her hips forward. Krista frowned slightly and grit her teeth in concentration; Ymir gasped and choked out a moan.

"What the.. Krista, fuck.." she groaned.

Krista grinned widely and once more tightened her inner walls on Ymir. The brunette bit her lip and narrowed her eyes. Her vision was hazy with pleasure and she swore she saw stars from all the heat and friction against her shaft. Krista slipped her hands under Ymir's shirt and gently scraped her nails along her muscular back. The tall girl shuddered and rolled her hips in time with her thrusts, laughing quietly when Krista whined and dug her nails into her shoulders.

"Ymir," Krista moaned, "I'm getting close.. go a bit faster, please- ohh.."

"Shit.. these damn lockers.. Krista we are not doing this again, you hear?"

Krista nodded and shut her eyes as she clung to her lover with all her might. It was sweltering by now and they were making so much noise with their activities. It wouldn't take much for someone to stumble upon them..

"Ah! I'm.." Krista bit into the joint area between Ymir's neck and shoulder as she came, body trembling. Ymir cursed and came as well, her essence filling Krista. They slumped against each other and smiled tiredly. They stayed like that for a while, listening to the harsh breaths and heart beats of their significant other. Ymir straightened - as much as she could with her neck bent over - and helped Krista clean up and fix her clothes. She buckled her pants up and opened the locker. Thankfully, no one was outside to witness her, and she pulled Krista along with her as well and shut the locker. Once she clipped the lock onto it, Krista took her hand and intertwined their fingers. It made Ymir smile and pull her close for a sweet kiss.

"That was nice," Krista sighed.

"Yeah, but now my neck hurts like hell," Ymir complained.

Krista rolled her eyes and bumped into Ymir. "You big baby.. I'll fix it for you."

"I'll just have to take you up on your offer.."

They laughed together and pushed the doors open to greet the warm breeze.

x

Note: Where I go to school, lockers are life-size, which is why they could fit inside :P


End file.
